Polymer Pen Lithography (PPL) is a recently developed molecular printing technique that employs an array of pyramidal, elastomeric tips to pattern soft organic and biologically-active inks onto surfaces with micrometer to nanometer scale feature dimensions. PPL has attracted wide interest because of its broad ink and substrate scope, the low-cost of the tip-arrays, the precise control over feature size and pattern design afforded by the piezoelectric actuators that move the tips, and the ability to form combinatorial arrays for applications including multiplexed detection and sensing. Many of the advantageous properties of this lithographic method arise directly from the array fabrication protocol and the chemical and mechanical properties of the elastomer. By curing poly(dimethyl siloxane) (PDMS) in a reusable master prepared by conventional photolithographic techniques, tip arrays mounted onto a glass support with as many as 107 tips are prepared in a process that takes approximately 2 days. To transfer inks from the tips to a surface, an aqueous meniscus forms that is a conduit for ink transport. As a result of the meniscus, there is a well-known linear relationship between the square root of the dwell-time that the tips rest on the surface during printing and the resulting feature diameter that is also common to another popular tip-based lithography strategy, dip-pen nanolithography (DPN). Unlike DPN, which uses the Si tips on the cantilevers of atomic force microscope (AFM) probes as a stylus, the elastomers employed in PPL deform upon contact with the surface, so the force applied between the tip array and the surface is another parameter that, in addition to dwell-time, can be manipulated to achieve nanoscale control over feature dimensions.
While the compressibility of the PDMS tips typically used for PPL provides increased control over feature dimensions, tip deformation may not always be desirable, and in particular may cause difficulties in leveling the tip array with respect to the surface. Because elastomeric tips deform upon contact with the surface, tips that do not contact the surface simultaneously produce features of different dimensions, which has led to elaborate strategies for leveling the tip array with respect to the surface. Hard-tip soft spring lithography (HSL), which employs Si tips mounted onto an elastomeric backing, and tip arrays composed of a dual-elastomer, developed by Zheng et al., minimize the effects of leveling by providing a soft backing that allows all the tips in unleveled arrays to contact the surface simultaneously when a large force is applied between the tips and the array. However, both HSL and dual-elastomer tip arrays involve multistep fabrication protocols that increase the cost and challenge of employing those new materials in the context of nanolithography. Thus, there remains a need for alternative materials that would retain the simplicity characteristic to the fabrication of PDMS tip arrays while preserving the control over feature dimensions and to understand the role of the mechanical properties of the different materials on PPL printing.